This invention relates to nickel base superalloys, and, more particularly to such alloys for use at about 2000.degree. F. and above, especially as single crystal shapes.
One important and continuing development which has enabled designers of gas turbine engines to provide more efficient and higher temperature operating aircraft engines is the evolution of the nickel based superalloy. The published art describes early turbine section components of such alloys as members having a predominantly multi-grained equiaxed microstructure. Later came the development of directional solidification of cast components leading to grain structures of multiple, axially elongated grains and then to single crystal structures.
Because the single crystal structures avoided grain boundaries, compositional elements added solely for grain boundary strengthening were eliminated. However, the formation in the microstructure of the single crystal of phases detrimental to the strength and integrity of the component made therefrom increased in importance as the intended application temperature increased. For example, it has been recognized that there can form in certain nickel base superalloys, including those designed for the manufacture of single crystal structures, a constituent which can result in loss of mechanical properties, particularly stress rupture life, after exposure to temperatures of at least about 1800.degree. F. Such a detrimental formation consists essentially of an orthorombic "P" phase which is a Type II topologically close-packed (TCP) phase, and a gamma phase, both dispersed in a gamma prime matrix, the TCP phase and the gamma phase forming needlelike structures within the blocky gamma prime matrix. This detrimental formation is hereinafter referred to as a Secondary Reaction Zone (SRZ).